1. Field of the Invention
The present invention relates to alloys which have excellent erosion resistance and stress corrosion cracking resistance and are suitable for use in apparatus, equipment, devices and parts susceptible to erosion due by a fluid, droplets and/or cavitation, such as erosion shields of steam turbines and valves, especially in application fields where erosion resistance and stress corrosion cracking resistance are both required.
2. Description of the Prior Art
Stellites, which are Co-Cr-W-C alloys excellent in erosion resistance, are now used in apparatus, devices, equipment and parts susceptible to erosion by a fluid, droplets, cavitation and/or the like, such as erosion shields for steam turbines and valve seats for piping, led by those employed in nuclear power plants. Stellites however have a high Co content and are hence costly. Moreover, when employed especially in nuclear power plants, Co is rendered radioactive, thereby posing the problem that people and other living creatures may be exposed to radiation.
To overcome these problems, the present inventor previously proposed, as Co-free alloys having excellent erosion resistance, the alloys disclosed in Japanese Patent Application Laid-Open Nos. 317652/1988 and 111844/1990. Describing the specific compositions of these alloys, the former are alloys consisting of 0.35-2.7% C, .ltoreq.2.5% Si, 10-25% Mn, 6-20% Cr, 0.5-11% V, .ltoreq.0.1% N, all by weight basis, and the balance being essentially Fe. They may additionally contain .ltoreq.3% Ni and/or .ltoreq.4% Mo. The latter are alloys having excellent erosion resistance and consisting of 0.9% &lt;C .ltoreq.1.7%, .ltoreq.2.5% Si, 10-25% Mn, 6-20% Cr, 3.7-7% V, .ltoreq.0.1% N, and either one or both of .ltoreq.5% W and .ltoreq.3% Ti, and the balance being essentially Fe.
The alloys disclosed in Japanese Patent Application Laid-Open Nos. 317652/1988 and 111844/1990 are usually employed after applying solution treatment at 1,150.degree. C. and then aging at 750.degree. C. As a result of a detailed investigation on the above alloys by the present inventors, it has become clear that alloys subjected to aging at 750.degree. C. are, despite of their excellent erosion resistance, insufficient in stress corrosion cracking resistance when employed in an environment tending to induce stress corrosion cracking, especially as an apparatus, device, equipment or part which is used to handle a salt-containing fluid.
It has however been found that the stress corrosion cracking resistance of these alloys can be improved without any reduction in erosion resistance when they are subjected to aging at a high temperature of 775-975.degree. C. subsequent to their solution treatment at a conventional temperature. The above heat treatment however involves the problem that it cannot bring about sufficient effects when high stress greater than about 40 kgf/mm.sup.2 is applied although improvements to stress corrosion cracking resistance are observed for stress less than about 40 kgf/mm.sup.2.